Terephthalic acid (TPA) is an important chemical intermediate which is used for the production of industrially significant products, including polyester polymers which can be used for fibre production and in the manufacture of bottles.
Current technology for the manufacture of terephthalic acid involves the liquid phase oxidation of paraxylene feedstock using molecular oxygen in a lower (e.g. C2–C6) aliphatic monocarboxylic acid, usually acetic acid, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine. In general, acetic acid, molecular oxygen in the form of air, paraxylene and catalyst are fed continuously into an oxidation reactor at elevated temperature and pressure. Air is added in amounts in excess of the stoichiometric requirements for full conversion of the paraxylene to TPA, to minimise formation of undesirable by-products, such as colour formers. The oxidation reaction is exothermic, and heat is removed by allowing the acetic acid solvent to vaporise. The corresponding vapor is condensed and most of the condensate is refluxed to the reactor, with some condensate being withdrawn to control reactor water concentration (two moles of water are formed per mole of paraxylene reacted).
The effluent, i.e. reaction product, from the oxidation reactor is a slurry of crude TPA crystals which are recovered from the slurry by filtration, washed, dried and conveyed to storage. The crystals are thereafter fed to a separate purification step or directly to a polymerisation process.
Generally, however, the terephthalic acid obtained is not sufficiently pure for direct use in polyester production since it contains, as major impurities, partially oxidised intermediates of terephthalic acid, particularly 4-carboxybenzaldehyde (4-CBA), although also p-tolualdehyde and p-toluic acid, along with other various colour-forming precursors and coloured impurities.
It is known that poor mixing in the oxidation reactor leads to undesirably high levels of the above-mentioned impurities in the final TPA product. An additional problem in the oxidation reactor is that detailed design features and operating conditions, e.g., temperature, catalyst concentration and residence time, can cause significant degradation of the solvent and precursor, which, in turn, can increase the cost of the operating process. A further important feature of the oxidation reactor is that solid crystals of crude TPA formed in the reactor must be adequately suspended to prevent choking of vessel walls and pipelines which in turn leads to unsafe operation of the process and frequent interruption for cleaning.
Vigorous agitation is known to reduce the above problems and Japanese patent no. 10316614 describes a multi-impeller agitator with the lower paddles larger than the upper paddles. However, such a system suffers from high initial cost and high operating cost. U.S. Pat. No. 5,211,924 describes a multi-agitator system in which the paddle type, size and location in the oxidation reactor reduce operating costs and reduce levels of impurity in the TPA product.
An alternative approach to reducing the levels of impurities in the crude TA product formed in the oxidation reactor has been to disperse the air and liquid feeds to the reactor more uniformly. JP 2000128824 describes a reactor whereby liquid feed is dispersed through a plurality of nozzles in the reactor, located at various points across the diameter or vertical height of the reactor, and matched with the flow of oxygen throughout the reactor. Such an arrangement prevents high concentrations of liquid feed, such high concentration being known to cause higher impurity formation. However, such a process is complex, the reactor is costly to fabricate and does not solve the problem of solid suspension.
WO02/092549 discloses an agitation system for TPA oxidation reactors consisting of an asymmetric radial turbine in combination with one or more down-pumping axial impellers. However, this potentially results in foaming in the reaction mixture which can result in carryover of reaction mixture into the reactor headspace and overheads equipment which leads to fouling and other operating problems.
It is therefore an object of this invention to provide an apparatus for producing TPA having decreased amounts of impurities, particularly coloured impurities. It is a further object of this invention to provide an apparatus having low equipment cost and low operating cost. It is a still further object of this invention to provide an apparatus in which the produced TPA is suspended in the solvent to prevent choking of the apparatus.